The host innate immune response, initiated by the sensing of non-self viral nucleic acid, determines the outcome of most diseases resulting from viral infection. Therefore, in order to effectively combat viral diseases, it is necessary to critially understand the control mechanisms of the innate immune responses. We have discovered that interferon (IFN)-stimulated gene Oligoadenylate Synthetase-like (OASL) can differentially control the host response against RNA and DNA viruses. This proposal will test the hypothesis that OASL differentially modulates the sensitivity and the outcome of innate immune signaling against RNA and DNA viruses and determine the distinct molecular mechanisms by which OASL controls innate immune signaling in vitro and in vivo. We have found that human OASL sensitizes the RNA-sensor, RIG-I by mimicking one of the RIG-I ligand polyubiquitin. Thus, OASL enhances RIG-I-signaling-mediated IFN induction, and exerts a strong antiviral activity against multiple RNA virus infections. Consequently, genetic ablation of OASL in human cells, or Oasl2 in mice reduces RIG-I-mediated IFN induction and enhances susceptibility to various RNA virus infection (Zhu et al. 2014, Immunity. 40:936-48). On the other hand, the OASL or Oasl2-deficient cells show enhanced IFN response to DNA-sensor, cGAS activation, and consequent lower DNA virus replication. Our initial results indicate that OASL through its direct interaction with cGAS, may negatively regulate the cGAS signaling. As some of the DNA viruses establish long-term infections, one of the major significances of this negative regulation of cGAS signaling by OASL can be to restrict inflammation during DNA virus infections. The goal of this proposal is to determine the molecular mechanisms of OASL-mediated modulation of innate immune signaling through three independent specific aims: (1) determine the molecular mechanism of RIG-I signaling enhancement, and changes of RIG-I properties by OASL; (2) determine the mechanisms and consequences of OASL-mediated modulation of IFN induction through the DNA-sensor cGAS; and (3) define the in vivo role of OASL in antiviral responses using two model RNA (VSV) and DNA (HSV) viruses. Thus, this study highlights the unique activity of human OASL and determines the mechanistic basis of its function that will guide the future development of therapeutic strategies against specific virus infection by targeting the OASL-pathway.